In recent years, an OLED display has been attracting attention as a flat display panel. The OLED display can be driven by a DC voltage, and hence its drive circuit can be simplified. In addition, the OLED display has advantages such as having no viewing angle dependence unlike a liquid crystal display, being bright due to its self-luminescence, and having a rapid response speed. At present, the OLED display is mainly used in a small portable appliance such as a mobile phone, and is expected to be applied to an ultra thin screen television in future. It should be noted that the OLED display mainly uses a system in which an active device such as a thin-film transistor (TFT) is arranged at each pixel for driving as is the case with the liquid crystal display.
The OLED display comprises, for example, two glass substrates, a negative electrode made of metal or the like, an organic light-emitting layer, a positive electrode made of ITO or the like, and a bonding material. An epoxy resin having low-temperature curing property or an organic resin-based bonding material such as a UV-curing resin may be used as the bonding material.
However, the organic resin-based bonding material has a drawback of being unable to block completely penetration of gas and moisture, though it has an advantage of being able to bond glass substrates to each other at low temperature. Thus, air tightness inside the resultant OLED display cannot be maintained and its organic light-emitting layer, which has low resistance to oxygen and moisture, is liable to degrade. Therefore, a failure that a display characteristic of the OLED display degrades time-dependently easily occurs, and when the OLED display is used over a long period, reliability of the display is liable to deteriorate.
In view of the foregoing, a sealing material containing glass powder may be used in place of the organic resin-based bonding material. The sealing material containing glass powder has excellent water resistance and is suitable for providing the air tightness inside the OLED display, in comparison to the organic resin-based bonding material.
However, the glass powder generally has a softening point of 300° C. or more, and hence it has been difficult to apply the glass powder to the OLED display. Specifically, when glass substrates are sealed to each other with the above-mentioned sealing material, it is necessary to put the whole OLED display in an electric furnace and fire it at a temperature equal to or higher than the softening point of the glass powder, thereby softening and flowing the glass powder. However, an active device used in an OLED display only has heat resistance to a temperature of about 120 to 130° C., and hence, when glass substrates are sealed to each other by this method, the active device is damaged by heat, resulting in degradation of a display characteristic of the resultant OLED display. In addition, an organic light-emitting material is also poor in heat resistance, and hence, when glass substrates are sealed to each other by this method, the organic light-emitting material is damaged by heat, resulting in degradation of a display characteristic of the resultant OLED display.
In view of the circumstances mentioned above, laser sealing has been studied in recent years as a method of sealing an OLED display. The laser sealing can locally heat only the sites that should be sealed, and hence glass substrates can be sealed to each other while thermal degradation of an active device and the like is prevented.
For example, Patent Literatures 1 and 2 describe that a front glass substrate and a back glass substrate in a display unit such as a field emission display are sealed to each other with laser radiation.